This invention relates generally to surface treatments for metals, and more specifically to surface treatments for titanium alloys.
High-speed commercial aircraft require a surface treatment for titanium (Ti) alloys that is both environmentally safe and durable under the conditions of supersonic flight. A number of pretreatment procedures for Ti alloys requiring multiple stages have been developed to produce a stable surface. These stages include degreasing, mechanical abrasion, chemical etching, and electrochemical anodizing. These treatments exhibit significant variations in their long-term stability, and the benefits of each step in these processes still remain unclear. Moreover, these multistep processes are expensive and time consuming and, because of the multiple steps involved, are prone to error. In addition, the chromium compounds often used in these chemical treatments are detrimental to the environment.
Recently, a chromium-free surface treatment for Ti alloy has been reported [F. L. Keohan and B. J. Hecox, Proceedings of the 21st Annual Meeting of the Adhesion Society (Savannah, Ga., 1998), p. 60], although this treatment is not designed for high temperature applications. Other chromium-free surface treatments for Ti alloys include those described in U.S. Pat. No. 5,939,197 (Blohowiak et al.), U.S. Pat. No. 5,869,141 (Blohowiak et al.), U.S. Pat. No. 5,849,140 (Blohowiak et al.) and U.S. Pat. No. 5,814,137 (Blohowiak et al.).
Some metal treatment processes entail the use of hydrogen peroxide to oxidize the surface of the metal. However, the use of hydrogen peroxide is undesirable because of its attendant fire and explosion hazards. In particular, hydrogen peroxide provides oxygen very rapidly to facilitate or initiate burning of surrounding combustibles.
A further drawback of many metal treatments is that they degrade after exposure to hot, humid environments of the type encountered by many aircraft. Thus, the performance of such treatments, while initially satisfactory, is seen to degrade over time in the field.
There is thus a need in the art for a simplified surface treatment for titanium alloys and other metals. There is also a need in the art for a chromium-free surface treatment for titanium alloys and other metals which can be used in high temperature applications. There is further a need in the art for a surface treatment for titanium and other metals which exhibits good long-term stability. There is also a need in the art for metal treatments that can withstand hot, humid environments without significant degradation.
These and other needs are met by the present invention, as hereinafter described.
In one aspect, the present invention relates to a method for treating the surfaces of metals, such as those comprising titanium alloys, and to the surfaces so treated. In accordance with the method, oxides are formed on the surface of the alloy using a two-step chemical process featuring the application of an acid followed by a base or oxidizing agent, and without mechanical abrasion. The method results in a high performance surface for a variety of applications. The method of the present invention is cost effective and relatively safe to use in a commercial application. In addition, it is chromium-free, and can be successfully used with a sol-gel coating to afford a strong adhesive bond after exposure to hot-wet environments.
The metal treatments of the present invention exhibit excellent durability upon exposure to hot, wet environments with approximately 90% retention of initial strengths. An oxide layer developed by this base treatment can be controlled by time, temperature, and concentration. The morphology and chemical composition of the oxide layer was investigated and revealed the formation of metal oxide compounds on the treated surface.
In another aspect, the present invention relates to a method for controlling the surface energy and morphology of a metal substrate. In accordance with the method, the substrate, which may comprise, for example, a titanium alloy, is treated with an acid followed by a base. The base treatment time and/or temperature are controlled so as to create a desirable surface for bonding.
The method for treating metal substrates in accordance with the present invention has a number of notable features and attributes. It provides a simple acid-base surface treatment process that does not require mechanical abrasion or the use of chromium compounds, and therefore achieves an environmentally safe and stable metal surface for bonding. The surface treatment process comprises only a two-step chemical process, namely, acid etching of the substrate (e.g., with sulfuric acid) followed by surface oxidation of the substrate (e.g., with an alkaline perborate treatment as the oxidizing agent). The acid etching is used to generate a fresh metal surface and the base treatment is used to form a stable oxide on the surface. Other features and attributes include:
1. The process is simple and cost effective because it can be achieved using a simple two-step chemical process. Most of existing procedures for surface pretreatment require multiple stages such as degreasing, mechanical abrasion, multiple chemical etching, electrochemical anodizing, and so on.
2. The process can be applied to any complex substrate, including thin metal sheet and honeycomb structures, since the conventional mechanical abrasion procedure can be replaced by sulfuric acid etching.
3. The process is chromium-free, and therefore much more environmentally friendly than many conventional treatment processes.
4. The process is safe and convenient to use, since perborate powder is used as an oxidizing agent (e.g., instead of hydrogen peroxide) to generate a fresh oxide layer.
5. The process can be easily controlled by variation in solution concentration, treatment time and/or temperature.
6. The process has been successfully used with a sol-gel coating to afford a strong adhesive bond, which exhibits excellent durability even after the bonded specimens are subjected to a harsh 72 hour water-boil or 2000 hour exposure at 177xc2x0 C.
7. The surface energy and morphology can be efficiently controlled by the base treatment time and temperature to create a desirable surface for bonding.
8. The process can be applied to any solid substrate titanium alloy, stainless steel, aluminum alloy, glass, copper, and so on to promote bonding in the areas of paint, coating, automotive, and aircraft applications. Degrees of acid etching and base oxidization can be readily adjusted depending on the solid substrate.
9. The process can be applied in the electronic industry as a safer treatment for silicon wafers.
10. The process can be applied to inorganic inclusions (reinforcing particles, fibers, fabrics, and platelets) in composites to create a fresh oxide surface before surface modification (sizing).